Hydrothermal Synthesis of Octadecahedral Hematite - American

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Hydrothermal Synthesis of Octadecahedral Hematite (α-Fe2O3) Nanoparticles: An Epitaxial Growth from Goethite (α-FeOOH) Ming Lin,*,† Liling Tng,†,‡ Tongyi Lim,†,‡ Meeling Choo,†,‡ Jia Zhang,§ Hui Ru Tan,† and Shiqiang Bai† †

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, S117602 Singapore ‡ School of Applied Science, Temasek Polytechnic, 21 Tampines Avenue 1, S529757 Singapore § Institute of High Performance Computing, A*STAR (Agency for Science, Technology and Research), S117528 Singapore S Supporting Information *

ABSTRACT: Driven by the demand for shape-controlled synthesis of α-Fe2O3 nanostructures and the understanding of their growth mechanism and shape-dependent properties, we report the synthesis of octadecahedral α-Fe2O3 nanocrystals with a hexagonal bipyramid shape by introducing F− anions in the solution. The hydrothermal growth process from hydrolysis of Fe3+ precursors involves three steps: the nucleation of akaganeite (β-FeOOH) nanorods, followed by the formation of goethite (α-FeOOH) crystals with acicular and twinned shapes, and a subsequent transformation into hematite (α-Fe2O3) nanoparticles. The phase transformation and growth of α-Fe2O3 particles from α-FeOOH follows dissolution of goethite and reprecipitation as hematite process. The initial nucleation of α-Fe2O3 particles was found to form epitaxially on goethite {001} surfaces due to a perfect lattice match between goethite {001} surface and hematite {001} planes. The structural relationship between goethite and hematite is G(020)//H(030) with G[100]//H[100]. The obtained α-Fe2O3 hexagonal bipyramid particles are enclosed by 12 {113} planes and six {104} facets. Since the twinned α-FeOOH particles are one of the typical shapes of intermediate goethite crystals, the nucleation of hematite particles on two twinned arms gives rise to the formation of twinned hematite particles. F− anions play an important role in the formation of α-Fe2O3 particles with a hexagonal bipyramid shape because high concentration of F− anions can stabilize the exposed {113} surfaces. The controlled synthesis of αFe2O3 nanoparticles with defined surfaces not only provides significant information on hematite surface structures and energies but also is critical to give the structure−property relationship for the application of hematite materials.

1. INTRODUCTION Nanostructured materials with well-controlled size, shape, and composition not only present unique and promising applications in many areas but also provide an avenue for fundamental investigation of their intrinsic physicochemical properties due to the performance of materials being strongly correlated to their size and surface structures. Hematite (αFe2O3) is an environmentally friendly semiconductor material with wide applications as catalysts, adsorbents, batteries, and pigments, etc.1−11 The general strategies for synthesizing hematite nanoparticles include three main approaches: direct oxidation of iron metal, hydrolysis of iron precursors, and phase transformation from other iron oxide phases. The phase transformation could occur through dehydration (loss of H2O), dehydroxylation (loss of OH), and oxidation processes.1,12 Thus, hematite particles with varieties of morphologies, such as polyhedrons, rhombohedrons, cubes, wires, tubes, rings, and rods, have been successfully synthesized over the past decades.7 Because of similar surface energies of various low index facets of hematite, the order of stability of surfaces can be changed easily by preferential adsorption of ionic species or slight alterations to reaction conditions.1 Therefore, facet-controlled © 2014 American Chemical Society

synthesis of hematite nanoparticles can be achieved through adsorption of anions and/or organic molecules on specific surfaces. Numerous additives have been investigated with respect to their effect on the surface stabilities and crystal shapes. For example, rhombohedral α-Fe2O3 nanoparticles enclosed by six {014} facets were obtained by direct hydrolysis of ferric precursors in water,13,14 ethanol,15 water−triphenylphosphine,16 water−ethylene glycol,5 and ethanol−ethylene glycol solutions.17 Hexagonal α-Fe2O3 nanoplates were synthesized with the assistance of sodium acetate in ethanol,18 in which {001} planes were dominantly exposed. In the presence of organic surfactants (oleylamine, oleic acid, and acetylacetone)19 or inorganic chelating agents (CTAB),20 αFe2O3 nanocubes bounded by two {014} and four {012} facets could be generated in solvothermal reactions. The addition of F− anions,21 1-butyl-3-methylimidazolium tetrafluoroborate,22 or formamide 23 resulted in the formation of α-Fe 2 O 3 nanoparticles with a hexagonal bipyramid shape. A similar morphology can also be grown using K3[Fe(CN)6], N2H4 and Received: February 28, 2014 Revised: April 11, 2014 Published: April 16, 2014 10903

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The morphological and structural analysis of the obtained nanoparticles were performed on a scanning electron microscope (JEOL-6700F), a FEI Titan 80/300 scanning/transmission electron microscope (TEM) (200 kV), and an X-ray diffractometer with Cu Kα1 radiation (λ = 1.5406 Å). To evaluate the influence of washing procedures on the analysis of particle morphologies, the synthesized Fe2O3 nanoparticles were collected by TEM copper grids both immediately after hydrothermal reactions and after thorough wash, respectively. No obvious changes to the size and shape of particles were observed after the washing process. The crystal structures and atom diagram on various surfaces were constructed by CaRine Crystallography 3.1. A total of 77 HAADF-STEM images were collected for electron tomography over a tilt range of −76° to 76° with 2° tilt step. The acquisition time for one 1024 × 1024 sized image was 40 s. The final tilt series was aligned using a cross-correlation method and reconstructed by the simultaneous iterative reconstruction technique (SIRT, 40 iterations) using Inspect3D, and the reconstructed 3D volume was visualized with Amira 4.1.26−28

sodium carboxymethyl cellulose mixtures via the hydrothermal process.24,25 In general, the synthesis of these α-Fe2O3 nanoparticles with defined crystal facets can be carried out by facile one-step hydrothermal/solvothermal reactions through fine-tuning of the experimental conditions. Many efforts have been made to control the growth of shape-controlled hematite particles for unique functional properties and potential applications. However, few literatures systematically studied the structures of particles, and details of how the hematite particles form are still unknown. For example, hexagonal bipyramid α-Fe2O3 nanoparticles have been synthesized and reported by several groups.21−25 Although these bipyramid particles possessed very similar or same morphologies, the 12 equiv exposed facets were assigned to {111},21 {101},25 {102},24 and {112}23four different family planes by different research groups, in contradiction to each other. The incorrect identification of surfaces will result in an inaccurate characterization and interpretation of the properties of the nanostructured αFe2O3 particles. In addition, a comprehensive examination of the structures of the nanoparticles, such as shape, pore structure, defects, and dislocations in the crystal, can reveal crucial information for understanding particle growth. This is critical for the synthesis of desired nanostructured materials with controlled composition, crystallinity, morphology, and physicochemical properties. Recently, we have reported the role of two ripening mechanisms (Ostwald ripening and oriented attachment) in the formation of α-Fe2O3 nanoparticles with various shapes.15 Oriented attachment through {104} facets of α-Fe2O3 nuclei led to the formation of flower-like α-Fe2O3 nanocrystals when Ostwald ripening process was suppressed. With the addition of water, Ostwald ripening process (dissolution−reprecipitation) would play an important role in the reaction and convert the assembled nanoflowers into 3D rhombohedral morphologies with well-defined edges and dominant {104} surfaces. In this paper, driven by the demand for shape-controlled synthesis of α-Fe2O3 nanostructures and shape-dependent properties, we report the synthesis of octadecahedral α-Fe2O3 nanocrystals with a hexagonal bipyramid shape by introducing F− anions in the solution. The morphological evolution of the nanostructures, role of F− anions, and corresponding growth process were discussed based on the insightful analysis of the shape and structure of the as-synthesized particles. The findings are of fundamental importance to understand the morphology and growth of α-Fe2O3 nanostructures. The better control of the shape evolution of hematite particles will provide direct correlation between exposed facets and facet-controlled properties in future study.

3. RESULTS Figure 1 shows the XRD patterns of nanoparticles prepared with 0.12 mL of NH3 solution and different amounts of NaF.

Figure 1. XRD patterns of the synthesized nanoparticles as a function of F− concentration: (a) 0, (b) 3.5, (c) 7.0, (d) 10.5, (e) 14, (f) 17.5, and (g) 35 mM.

The pH values were measured to be around 1 for all solutions after hydrothermal reaction, indicating that an acidic medium was formed by the forced hydrolysis of ferric ions and production of H+ and iron oxide precipitates. The XRD results revealed that pure α-Fe2O3 crystals with a rhombohederal structure (JCPDS 34-1266) were obtained when the concentration of F− anions was lower than 10.5 mM, as shown in Figure 1a−d. With further increase in the concentration of F− anions, the crystalline structure of particles changed to a mixture (Figure 1e) of α-Fe2O3 and goethite (α-FeOOH) and finally to a pure α-FeOOH phase with an orthorhombic structure (JCPDS 29-0713). Since the space group of goethite has been moved from Pbnm to Cmcm,1,29 the orthorhombic space group of goethite has been indexed with cell parameters of a = 9.956 Å, b = 3.022 Å, and c = 4.608 Å in this paper, where the lattice constant along the c-axis of goethite was coincident with 3 times that along the c-axis of hematite. The

2. EXPERIMENTAL SECTION Octadecahedral α-Fe2O3 nanoparticles with a hexagonal bipyramid shape were synthesized by the hydrothermal method. 1 mmol of ferric chloride hexahydrate (FeCl3·6H2O) was dissolved in 30 mL of deionized (DI) water. Different amounts of sodium fluoride and ammonia (12−14%, 0−5 mL) were then mixed with the starting solution. The mixture was stirred for 10 min at room temperature and then transferred into a 50 mL Teflon lined autoclave (Fisher Scientific). The autoclave was heated at 180 °C for 24 h. After reaction, the precipitate was collected and washed with ethanol and DI water. The final α-Fe2O3 powders were obtained by drying the precipitates in vacuum oven overnight. 10904

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more uniform as the concentration of NaF increases. Further increase in F− anions favors the growth of mixed hematite and goethite particles. It is also interesting to note that hematite particles are observed to nucleate and grow on the surface of goethite nanorods. To get more insight into the shape and structural evolution of hydrolyzed products, α-Fe2O3 bipyramid (Figure 2d) and the hybrids (Figure 2e) were analyzed by highresolution TEM (HRTEM) and electron tomography. SEM images in Figure 3a,b show the hematite nanoparticles with a hexagonal bipyramid shape, which are enclosed by 12

characteristic (012) peak of hematite and (101) peak of goethite are highlighted in Figure 1. Consistent with the evolution of crystalline structures, the nanoparticles exhibited distinctly different shapes and morphologies by varying the amount of NaF added. Figure 2 shows

Figure 3. Structural analysis of α-Fe2O3 hexagonal bipyramid particles. (a, b) SEM image, twinned α-Fe2O3 particles were highlighted by arrows; (c) top view of the reconstructed 3D volume from electron tomography; (d−g) TEM image, HAADF−STEM image, corresponding diffraction pattern and HRTEM image showing the {113} surfaces parallel to the electron beam; (h−k) TEM image, HAADF−STEM image, corresponding diffraction pattern and HRTEM image showing the {104} surfaces parallel to the electron beam when the same particle was tilted 30°.

Figure 2. SEM images of shape evolution of α-Fe2O3 nanoparticles as a function of F− concentration: (a) 0, (b) 3.5, (c) 7.0, (d) 10.5, (e) 17.5, and (f) 35 mM. White arrows in (e) and (f) indicate the presence of α-Fe2O3 nanoparticles on α-FeOOH.

the SEM images of nanoparticles obtained with 0, 0.35, 0.7, 10.5, 17.5, and 35 mM F−. Without addition of NaF, α-Fe2O3 nanoparticles were spherical (Figure 2a). The addition of NaF slowly changed the morphology from spheres to hexagonal bipyramids, as demonstrated in Figure 2b,c. When the NaF concentration was increased to 10.5 mM, α-Fe2O3 nanoparticles with a hexagonal bipyramid shape were synthesized with an average size of 300 nm, as shown in Figure 2d. The increase of NaF concentration to 17.5 mM led to the formation of mixed α-Fe2O3 bipyramid particles and α-FeOOH particles (Figure 2e). α-FeOOH particles demonstrated typical acicular and twinned shapes, which were depicted in an SEM image recorded from a large area in the Supporting Information. Such structures were the characteristic shapes of goethite particles, which has been fully analyzed by Schwertmann.1 By further increasing the [F−] to 35 mM, majority of the products were αFeOOH rods, where tiny α-Fe2O3 nuclei were occasionally observed on the surface of α-FeOOH rods, as indicated by arrows in Figure 2f. Owing to the low concentration and small crystallites, the reflections from these α-Fe2O3 nucleus are substantially weaker than the main reflections of FeOOH and probably lower than the detection limit of XRD, which cannot be observed in XRD pattern (Figure 1g). Thus, it is clearly shown that the concentration of F− anions plays an important role in controlling the size, shape, and crystal structures of the nanoparticles which were hydrolyzed from ferric ions. Low concentration of F− results in the formation of hematite nanoparticles, where the shape becomes

identical side surface and 6 top facets. This shape was further confirmed by the electron tomography analysis in Figure 3c. A rough surface shown in Figure 3c was caused by missing wedge artifacts due to the limited tilt angle in TEM.26,27 Interestingly, a careful examination of SEM images revealed the formation of twin hematite particles (highlighted by arrows), which were composed of two identical bipyramid particles. The crystallographic orientation of each exposed surface was determined through the analysis of diffraction and highresolution TEM images. It should be noted that the lattice fringes observed in high-resolution TEM images usually cannot be assigned to the surface facets except when the exposed surfaces are parallel to the electron beam.15 Therefore, the nanoparticles were tilted in TEM until the surface was parallel to the beam. When the α-Fe2O3 bipyramid particle was tilted along the [1−10] direction (Figure 3d,e), four side facets were parallel to the electron beam. Selected area diffraction pattern (Figure 3f) taken from this particle showed a single crystalline structure without presence of twins and stacking faults. The four exposed side planes could be indexed to (1̅1̅3), (113̅), (113), and (1̅1̅3̅) planes, respectively. The spacing of lattice planes parallel to the surface was measured to be 0.22 nm, corresponding to α-Fe2O3 (113) plane distance (Figure 3g). When the particle was tilted 30° from [1−10] to [100] zone axis, two top surfaces have become parallel to the electron 10905

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Increasing the concentration of F− anions resulted in the formation of α-FeOOH and nucleation of α-Fe2O3 hexagonal pyramid at the center of acicular α-FeOOH particles, as shown in Figure 5a,e. The acicular α-FeOOH particles grew along the [010] direction and showed a diamond-like cross section. The angle measured between two top surface facets was around 130°, in agreement with the angle between {101} planes. This is consistent with the typical shape of the acicular goethite crystals which are enclosed by {101} facets.1,30 Figure 5b−d presents a side-view TEM image of a hybrid α-Fe2O3−αFeOOH particle and the corresponding microdiffraction patterns. The diameter of the diffraction aperture was about 110 nm, which could cover the two parts separately. The indices of goethite and hematite are indicated in the patterns. The diffraction pattern of α-FeOOH was recorded parallel to [100] zone axis. It confirmed the elongation of acicular goethite along the ⟨010⟩ direction. Hematite has displayed a similar diffraction pattern to that of α-FeOOH, suggesting a possible epitaxial relationship between α-Fe2O3 and α-FeOOH particles. The structural relationship was interpreted as G(020)//H(030) with G[100]//H[100]. This crystallographic relationship was also observed during the thermal dehydroxylation and growth of hematite on goethite at high temperature.1,29,31 This relationship can be better understood through the schematic drawing of α-Fe2O3 hexagonal pyramid on an acicular αFeOOH particle in Figure 5f. This is the first experimental evidence which clearly shows how the hematite grows epitaxially on goethite in solution. With this structural relationship, the H(001) plane was on top of G(001) facets and H(1,1,12) plane grown on majorly exposed G(101) facets. Although the G(001) facets were not the major exposed surface on acicular α-FeOOH particles, it was found that hematite preferred to nucleate on these surfaces first (Figure 5a). Figure 5g,h presents the simulated atomic structures of G(001) and H(001) planes. The square indicates a unit cell of oxygen anion on the top surface. It can be found that a unit cell with a parameter of 10.08 × 8.73 nm on H(001) matched well with the unit cell on G(001) with a dimension of 9.96 × 9.06 nm. It was elongated with a factor of 1.2% in the G[100] direction and contradicted of 3.8% in the G[010] direction when H(001) plane epitaxially grows on G(001) facets.

beam and can be indexed as the (014̅ ) and (014)̅ planes from the diffraction pattern (Figure 3j). A high-resolution TEM image obtained from the black square in Figure 3k showed a perfectly resolved lattice spacing of 0.27 nm, which corresponded to hematite (014) planes. The strips observed in the bright field image (Figure 3d,h) were caused by the wedgelike shape of the particles, where direct and diffracted beams oscillated in a complementary way with continuous change of the wedge thickness.15 This phenomenon roughly differentiated the circumference of particles either with the surface facets parallel to the electron beam or with the wedgelike edges in TEM. More evidence about the surface facets parallel to the electron beam was brought by HAADFSTEM images. HAADF-STEM images are dominated by mass−thickness contrast and are much less sensitive to diffraction contrast and Fresnel fringes. Thus, orientation of the particles and surfaces can be better and easily understood in HAADF-STEM images in Figure 3e,i. Combining the SEM, TEM, and electron tomography analysis, the α-Fe2O3 nanoparticles can be unambiguously assigned to a single crystal structure with a hexagonal bipyramid shape, which is an octadecahedron enclosed by 12 identical {113} side surfaces and six {104} top surfaces. This is in good agreement with theoretical structures of hematite with a rhombohedral structure (space group R3̅C), where {10x} family planes have 3-fold symmetry while {11x} family planes have 6-fold symmetry. The schematic drawings of α-Fe2O3 hexagonal bipyramid and plane indices are shown in transparent (Figure 4a) and solid (Figure 4b) form.

Figure 4. Schematic diagram showing the shape of a hexagonal bipyramid particle: (a) side view; (b) top view with indices.

Figure 5. Structural analysis of hematite-on-goethite particles. (a) SEM image of particles obtained with 35 mM F−; (b) TEM image of a hematiteon-goethite particle; (c) diffraction taken from the middle goethite; (d) diffraction taken from the bottom hematite part; (e) SEM image of particles obtained with 17.5 mM F−; (f) a schematic diagram showing the shape of particle in (e); (g) atomic arrangement on goethite (001) plane; (h) atomic arrangement on hematite (001) plane. Red: Fe; blue: O; orange: top Fe atoms; cyan: H. 10906

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The majority exposed surface of acicular goethite was the {101} plane. The arrangements of oxygen atoms on G(101) and H(1,1,1,12) are shown in Figures 6a and 6b, respectively.

Figure 8. XRD patterns of the iron compounds obtained at different reaction periods: (a) 2, (b) 4, (c) 6, (d) 8, and (e) 24 h. A, G, and H represent akaganeite, goethite, and hematite phase, respectively.

Figure 6. (a) Atomic arrangement on goethite (101) plane. (b) Atomic arrangement on hematite (1,1,12) plane. Red: Fe; blue: O; cyan: H; orange: top O atoms.

nanorods. The lattice spacing of crystalline nanowires measured in the diffraction pattern were 0.335, 0.256, 0.227, 0.193, 0.162, and 0.150 nm, respectively, corresponding to an akaganeite (βFeOOH) structure (Figure S2 in the Supporting Information). It is worth to note that unlike the formation of both 2-line ferrihydrite particles and akaganeite nanorods during the initial hydrothermal hydrolysis in ethanol solution,15 higher dissolution rate of 2-line ferrihydrite in water led to the fast formation of akaganeite β-FeOOH nanorods. The intermediate β-FeOOH nanowires were not stable and had undergone further hydrolysis to form more stable αFeOOH nanorods through the dissolution−reprecipitation process. The thicker acicular α-FeOOH particles are indicated by the white arrows in Figure 7b. Some α-Fe2O3 hexagonal bipyramid particles were generated by surrounding an αFeOOH nanorods at the center. XRD pattern in Figure 8b shows the coexistence of α-FeOOH and α-Fe2O3 crystal phases. With reaction time prolonging from 4 h (Figure 7c) to 6 h (Figure 7d), all β-FeOOH nanowires have been consumed to form acicular α-FeOOH particles. The α-Fe2O3 hexagonal bipyramid particles became larger and inner α-FeOOH substrate diminished or totally disappeared. After 8 h of reaction, pure α-Fe2O3 hexagonal bipyramid particles were obtained with complete consumption of α-FeOOH particles, as shown in Figure 7e,f. This was confirmed by XRD analysis in which only rhombohederal α-Fe2O3 phase was observed. Twenty-four hour hydrothermal reaction yields α-Fe2O3 hexagonal bipyramid particles with uniform size through an Ostwald ripening process. The large particles grow larger with better crystalline structures at the consumption of small particles. It is interesting to note that a hole was left after the dissolution of α-FeOOH rod during hydrothermal reactions. Those holes will eventually disappear with the continuous reaction through Oswald ripening. The proportion of particles with holes decreased from ∼25% (8−12 h) to ∼14% after 24 h and ∼5% after 72 h reaction.

Comparing with G(001) and H(001) atomic arrangement, the slightly distorted unit cell on the H(1,1,12) would yield higher strain and stress in two orthogonal directions. This successfully explained the initial nucleation of hematite particle preferentially occurred on the goethite (001) planes (center of acicular goethite) rather than on the generally exposed {101} surfaces. The tiny nucleus on goethite surface will eventually become an α-Fe2O3 hexagonal bipyramid particle through the dissolution of goethite and precipitation as hematite process. To further explore the growth process of α-Fe2O3 hexagonal bipyramid particles, we have conducted time-dependent experiments to track the structural evolution of iron oxide particles during hydrothermal process (Figures 7 and 8). The concentration of F− anions was maintained at 10.5 mM. In Figure 7a, short and thin nanowires (50−100 nm long and 10 nm wide) have been observed after 1 h reaction at 180 °C. Since the sample quantity was not enough for XRD analysis, diffraction pattern was used to examine the structures of

4. DISCUSSION The hydrolysis of ferric ions to form α-Fe2O3 hexagonal bipyramid particles with addition of F− anions involves three stages: the initial nucleation of akaganeite β-FeOOH nanorods, followed by the formation of goethite α-FeOOH crystals with acicular and twinned shapes and a subsequent phase transformation into hematite α-Fe2O3. Careful structural examination indicates that the obtained α-Fe2O3 particles has a

Figure 7. TEM images showing the size and shape evolution of αFe2O3 nanoparticles as a function of time: (a) 1, (b) 2, (c) 4, (d) 6, (e) 8, and (f) 24 h. 10907

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microsized goethite crystals remained unchanged. In this study, the acidity of solution is influenced by three parameters: the hydrolysis reaction, addition of ammonia, and concentration of F−.3 On one hand, the hydrothermal reactions in an acidic condition favor goethite formation and then convert them into hematite crystals. Pure hematite crystals are only obtained in a narrow pH window (1.25−1.30) at 180 °C. The increasing pH through adjustment with NH3 solution yields mixtures of hematite and goethite (Figure S3b in the Supporting Information), suggesting a lower dissolution−reprecipitation process of goethite at higher pH. A stronger acidic solution ( 11).3 Goethite is one of the stable phases and is less reactive. Consequently, it has been reported that only the goethite nanoneedles partially convert to hematite with hydrothermal reaction at 160−180 °C in 1 week while

5. CONCLUSION In summary, α-Fe2O3 hexagonal bipyramid particles have been successfully synthesized with addition of F− anions. The formation of such structures from the hydrolysis of ferric ions involves three steps. Nucleation of akaganeite β-FeOOH nanorods is occurred first and followed by formation of goethite α-FeOOH crystals with acicular and twinned shapes. α-Fe2O3 nucleates on goethite {001} surfaces epitaxially due to the minimum strain and stress generated on interface. The fully grown α-Fe2O3 hexagonal bipyramid particles are enclosed by 12 {113} planes and six {104} facets. The phase transformation and growth of α-Fe2O3 particles follows dissolution of goethite and reprecipitation as hematite process. We believe this is a representative and most reasonable pathway for the phase transformation process from goethite to hematite in solution. The controlled synthesis of α-Fe2O3 nanoparticles with defined surfaces is critical to give the structure−property relationship for the application of hematite materials.33,34 Surface anisotropy is a defining factor on the equilibrium shape of crystals. The adsorption of foreign anions can stabilize the unsaturated surface bonds, defects, and steps, thus changing the order of the stability of surfaces. The change of surface energy induced by the presence of adsorbents is the driving force to the new morphologies. In this paper, it is found that {113} facets adsorbed with F− and {104} facets stabilized by OH− are the two most stable surfaces among hematite surfaces. The proportion of {113} and {104} planes is significantly affected by the concentration of anion concentration in solution. Unfortunately, we cannot provide experimental results on surface energies in this work. A theoretical simulation will be carried out in future to provide insightful information on 10908

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adsorption sites and binding energy of foreign anions and thus give surface energies with adsorption of them.

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ASSOCIATED CONTENT

S Supporting Information *

SEM image of mixture of α-Fe2O3 and α-FeOOH obtained with 17.5 mM F−; diffraction pattern of β-FeOOH nanorods after 1 h reaction; SEM images of α-Fe2O3 nanoparticles obtained with different acidity. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Tel 65-6874 5374, Fax 656874 4778 (M.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of IMRE (IMRE/12-1P0907, IMRE/131C0435) is gratefully acknowledged. The authors thank Mr. Nathaniel Too Sheng Hua from Singapore Catholic Junior College for the assistance on the experiments.

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REFERENCES

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